Optical fiber from the prior art can be arranged as follows for the purpose of measuring pressure. The optical fiber, which has a core, is provided with elements that apply constant anisotropic stress on the core. The core undergoes a deformation so that so that the core exhibits birefringence. Birefringence means that there are two possible modes of light propagation with two different propagation velocities. Hence, the core exhibits two different indices of refraction. In terms of magnitude, the birefringence can be defined as the difference between the two indices of refraction. The optical fiber from the art is further provided with a structure that converts isotropic external pressure into pressure-dependent anisotropic stress applied to the core. The isotropic external pressure affects the deformation of the core. Accordingly, the birefringence varies with isotropic external pressure that is applied to the optical fiber.
There are various ways to measure external pressure on the basis of the birefringence that the core of an optical fiber exhibits. According to one technique, the core is provided with a so-called fiber Bragg grating. As a result of the birefringence, the fiber Bragg grating has a spectral reflectivity response that comprises two peaks at two slightly different frequencies. That is, there is a frequency difference between the two peaks. The frequency difference is a function of the birefringence, and hence a function of the external pressure that is applied to the optical fiber. The external pressure can thus be determined on the basis of the frequency difference between the two peaks of the spectral reflectivity response, which the fiber Bragg grating provides.
U.S. Pat. No. 5,841,131 describes a fiber optic pressure transducer that includes a core which is provided with one or more gratings. The core is provided with cladding that has a pair of birefringence inducing rods, as well as a pair of longitudinal side holes. The side holes convert isotropic pressure forces to anisotropic forces. The stress-inducing rods are arranged with their longitudinal axes lying in a first plane. The longitudinal axes of the side holes lie in a second plane, which is substantially orthogonal to the first plane.
U.S. Pat. No. 5,309,540 discloses a hollow fiber with, over the full length of the fiber and between each fiber channel and the core, at least one stress zone to create an anisotropic stress distribution which subject the fiber core to a tensile stress. The fiber can be used as a pressure sensor or a pressure transducer.